The present invention relates generally to semiconductor devices and, more particularly, to a method and apparatus for fabricating submicron layers of Group nitride semiconductor materials.
III-V compounds such as GaN, AlN, AlGaN, InGaN, InAlGaN, and InGaAlBNPAs have unique physical and electronic properties that make them ideal candidates for a variety of electronic and opto-electronic devices. In particular, these materials exhibit a direct band gap structure, high electric field breakdown, and high thermal conductivity. Additionally, materials such as InxAl1-xGaN can be used to cover a wide range of band gap energies, i.e., from 1.9 eV (where x equals 1) to 6.2 eV (where x equals 0).
Until recently, the primary method used to grow Group III nitride semiconductors was metal organic chemical vapor deposition (MOCVD) although other techniques such as molecular beam epitaxy (MBE) have also been investigated. In the MOCVD technique, III-V compounds are grown from the vapor phase using metal organic gases as sources of the Group III metals. For example, typically trimethylaluminum (TMA) is used as the aluminum source and trimethylgallium (TMG) is used as the gallium source. Ammonia is usually used as the nitrogen source. In order to control the electrical conductivity of the grown material, electrically active impurities are introduced into the reaction chamber during material growth. Undoped III-V compounds normally exhibit n-type conductivity, the value of the n-type conductivity being controlled by the introduction of a silicon impurity in the form of silane gas into the reaction chamber during growth. In order to obtain p-type material using this technique, a magnesium impurity in the form of biscyclopentadienymagnesium is introduced into the reactor chamber during the growth cycle. As Mg doped material grown by MOCVD is highly resistive, a high temperature post-growth anneal in a nitrogen atmosphere is required in order to activate the p-type conductivity.
Although the MOCVD technique has proven adequate for a variety of commercial devices, the process has a number of limitations that constrain its usefulness. First, due to the complexity of the various sources (e.g., trimethylaluminum, trimethylgallium, and biscyclopentiadienylmagnesium), the process can be very expensive and one which requires relatively complex equipment. Second, the MOCVD technique does not provide for a growth rate of greater than a few microns per hour, thus requiring long growth runs. The slow growth rate is especially problematic for device structures that require thick layers such as high voltage rectifier diodes that often have a base region thickness of approximately 30 microns. Third, n-type AlGaN layers grown by MOCVD are insulating if the concentration of AlN is high ( greater than 50 mol. %). Accordingly, the concentration of AlN in the III-V compound layers forming the p-n junction is limited. Fourth, in order to grow a high-quality III-V compound material on a substrate, the MOCVD technique typically requires the growth of a low temperature buffer layer in-between the substrate and III-v compound layer. Fifth, generally in order to obtain p-type III-V material using MOCVD techniques, a post-growth annealing step is required.
Hydride vapor phase epitaxy or HVPE is another technique that has been investigated for use in the fabrication of III-V compound materials. This technique offers advantages in growth rate, simplicity and cost as well as the ability to grow a III-V compound layer directly onto a substrate without the inclusion of a low temperature buffer layer. In this technique III-V compounds are epitaxially grown on heated substrates. The metals comprising the III-V layers are transported as gaseous metal halides to the reaction zone of the HVPE reactor. Accordingly, gallium and aluminum metals are used as source materials. Due to the high growth rates associated with this technique (i.e., up to 100 microns per hour), thick III-V compound layers can be grown.
The HVPE method is convenient for mass production of semiconductor devices due to its low cost, flexibility of growth conditions, and good reproducibility. Recently, significant progress has been achieved in HVPE growth of III-V compound semiconductor materials. AlGaN, GaN and AlN layers have been grown as well as a variety of structures using this technique. Since this technique does not require low temperature buffer layers, a variety of novel device structures have been fabricated, such as diodes with n-GaN/p-SiC heterojunctions. Furthermore, p-type layers have recently been produced using HVPE, thus allowing p-n or p-i-n junction devices to be fabricated.
In order to fully utilize HVPE in the development and fabrication of III-V compound semiconductor devices, thin layers must be produced, on the order of a micron or less. Conventional HVPE techniques have been unable, however, to grow such layers. As a result, the potential of the HVPE technique for fabricating devices based on Group III semiconductors has been limited.
Accordingly, what is needed in the art is a method and apparatus for growing submicron Group III nitride compounds using HVPE techniques. The present invention provides such a method and apparatus.
The present invention provides a method and apparatus for fabricating thin Group III nitride layers as well as Group III nitride layers that exhibit sharp layer-to-layer interfaces.
According to one aspect of the invention, a method and apparatus for fabricating multi-layer Group III nitride semiconductor devices in a single reactor run utilizing HVPE techniques is provided. Preferably an atmospheric, hot-walled horizontal furnace is used. Sources (Group III metals, Group V materials, acceptor impurities, donor impurities) are located within the multiple source zones of the furnace, the sources used being dependent upon the desired compositions of the individual layers. Preferably HCl is used to form the necessary halide metal compounds and an inert gas such as argon is used to transport the halide metal compounds to the growth zone where they react with ammonia gas. As a result of the reaction, epitaxial growth of the desired composition occurs. By controlling the inclusion of one or more acceptor impurity metals, the conductivity of each layer can be controlled.
In at least one embodiment of the invention, the reactor includes one or more gas inlet tubes adjacent to the growth zone. By directing the flow of gas (e.g., an inert gas) generally in the direction of the substrates, epitaxial growth can be disrupted. The flow of gas can be directed at the substrate or in a direction that simply disrupts the flow of reactive gases such that epitaxial growth is halted.
In at least one embodiment of the invention, the reactor includes both a growth zone and a growth interruption zone. One or more gas inlet tubes direct a flow of gas (e.g., an inert gas) towards the growth interruption zone, thereby substantially preventing any reactive gases from entering into this zone. In use, after the growth of a layer is completed, the substrate is transferred from the growth zone to the growth interruption zone. The temperature of the substrate is maintained during the transfer and while the substrate is within the growth interruption zone, thus preventing thermal shock. While the substrate is within the growth interruption zone, the growth zone is purged and the sources required for the next desired layer are delivered to the growth zone. Once the reaction stabilizes, the substrate is returned to the growth zone. This process continues until all of the required device layers have been grown.
In at least one embodiment of the invention, the reactor uses a slow growth rate gallium source. The slow growth rate gallium source has a reduced gallium surface area. By reducing the surface area, there is less gallium available to react with the halide reactive gas. As a result, less gallium chloride is produced and fine control of the amount of gallium chloride delivered to the growth zone is possible.
In at least one embodiment of the invention, the reactor includes both a conventional gallium source and a slow growth rate gallium source. The slow growth rate gallium source dramatically reduces the surface area of the gallium exposed to the halide reactive gas, resulting in the production of less gallium chloride. Due to the low production levels, finer control of the amount of gallium chloride delivered to the growth zone is possible in contrast to the conventional source. Accordingly, a device can be fabricated during a single furnace run that includes both thick layers (i.e., utilizing the conventional gallium source) and thin layers (i.e., utilizing the slow growth rate gallium source).
In at least one embodiment of the invention, the reactor includes a conventional gallium source, a slow growth rate gallium source, one or more growth zones, and at least one growth disruption zone. The conventional gallium source is used in the fabrication of thick layers; the slow growth rate gallium source is used in the fabrication of thin layers; and the growth disruption zone is used to achieve fine control over layer composition and layer interfaces. The growth interruption zone uses one or more gas inlet tubes to direct a flow of gas towards the growth interruption zone, thereby substantially preventing any reactive gases from entering into the zone.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.